Acid-hardening self-hardening molds are generally each produced by: adding, to refractory particles such as silica sand, a binder for making foundry molds containing an acid-hardening resin, and a hardener including a sulfonic acid, sulfuric acid, phosphoric acid, or some other acid; mixing these components; filling the resultant mixed sand composition into an original pattern such as a wooden pattern; and then hardening the acid-hardening resin. As the acid-hardening resin, for example, a furan resin, a phenolic resin or the like is used. As the furan resin, for example, the following is used: furfuryl alcohol, furfuryl alcohol/urea-formaldehyde resin, furfuryl alcohol/formaldehyde resin, furfuryl alcohol/phenol/formaldehyde resin, or some other known modified furan resin. The resultant mold is used at the time of casting for a mechanical component casting, a construction machine component, an automobile component, or some other casting.
An item important for making the above-mentioned foundry mold or using the foundry mold to cast into a desired casting is to make an odor less bad at the time of the casting. The odor at the casting time is mainly sulfur dioxide gas originating from the curing agent component. In order to improve the working environment in casting factories, it is necessary to decrease the amount of generated sulfur dioxide gas as much as possible.
As a method for solving this problem, conceivable is a method of decreasing the content by percentage of sulfonic acid in the curing agent composition. However, it is inevitable that the foundry mold hardening speed is not lowered by the decrease.
Thus, curing agent compositions are disclosed which are each a mixture of sulfonic acid and another acid that does not contain sulfur to decrease the content by percentage of sulfonic acid without lowering the mold hardening speed (Patent Documents 1 and 2).
Moreover, a binder composition containing calcium hydroxide is disclosed for restraining the generation of sulfur dioxide gas (Patent Document 3).